1. Field of the Invention
The present invention relates to an apparatus and method for acquiring a preamble in an orthogonal frequency division multiple access (OFDMA) mobile terminal, and more particularly, to an apparatus and method for acquiring a preamble by correlating a received preamble symbol with two types of pseudo-noise (PN) codes having different lengths.
2. Discussion of the Background
The portable Internet, wireless broadband (WiBro), which is in the spotlight as a substitute for third generation (3G) wireless mobile communication technology based on Wideband Code Division Multiple Access (WCDMA), CDMA 2000, and the like, may offer solutions to problems of the third generation mobile communication technology, such as the limit of transmission speed.
The portable Internet, referred to as a 3.5 generation (3.5G) mobile communication technology, has advantages of mobility, which is provided in the third generation wireless mobile communication, and a high data transmission speed of broadband Internet.
The portable Internet adopts an orthogonal frequency division multiple access (OFDMA) system using a transmission band of about 100 MHz from a frequency band between 2.3 GHz and 2.4 GHz, and even when a user is traveling at a speed of about 60 km/hour, the portable Internet may support stable data transmission of more than 1 Mbps, which is different from the conventional third generation mobile communication technology. Accordingly, the portable Internet may be capable of simultaneously providing various types of services, and of transmitting multimedia data in real time.
OFDMA, which is a multi-access method adopted by the portable Internet, designates a system where a single channel carrier band is divided into many sub-channel carriers, which are also referred to as subcarriers, and users are allocated to a different group of valid subcarriers.
A connection between a mobile base station and a mobile terminal according to the OFDMA system includes an upstream channel corresponding to data transmission from the mobile terminal to the mobile base station, and a downstream channel corresponding to data transmission from the mobile base station to the mobile terminal. An upstream channel and a downstream channel transmit data by a frame unit, which includes a plurality of symbols. Here, a first symbol of each frame corresponds to a preamble, and the preamble is used for searching a cell and for performing base station identification and initial synchronization of a portable Internet terminal. Specifically, a cell searching process for searching for a cell of the portable Internet terminal and identifying a target base station with which to establish a wireless connection may frequently occur when supplying power to the portable Internet terminal or when performing a hand-off due to a portable Internet terminal's movement between cells. Also, the cell searching process should be performed quickly to ensure and maintain quality of service.
According to the Institute of Electrical and Electronics Engineers (IEEE) 802.16d/e standard, each frequency band includes 1,024 subcarriers. Of these 1,024 subcarriers, 172 subcarriers are guard band subcarriers, and 852 subcarriers are utilized for data transmission. Each base station is allocated with a plurality of subcarriers for each designated segment, and transmits a plurality of data symbols using the allocated subcarriers. Also, according to the IEEE 802.16d/e standard, each of 284 subcarriers in a group of 852 subcarriers, which are included in each of the frequency bands, are allocated among 3 segments. Accordingly, a single OFDMA symbol includes a bit string which is transmitted via 284 subcarriers.
FIG. 1 is a diagram illustrating a preamble transmission structure for each segment in a frequency domain. As shown in FIG. 1, the preamble, a type of the OFDMA symbol, is transmitted via 284 subcarriers, which are regularly arranged at intervals of 3 subcarriers. In FIG. 1, only a segment number of ‘0’ is described.
A preamble signal, which is transmitted from the base station, includes a unique bit string allocated to each base station. The mobile terminal receives the transmitted preamble signal, and compares the received preamble signal with a PN code generated in the mobile terminal, and thereby identifies the unique bit string contained in the preamble signal.
When comparing the preamble signal with the PN code, a correlation in the frequency domain is generally used. Specifically, a frequency domain preamble signal is correlated with a plurality of PN codes, and when a correlation value is greater than a predetermined threshold value, a bit string of a corresponding PN code is determined to be equal to the bit string of the received preamble signal. The above-described process is referred to as a preamble acquisition process.
FIG. 2 is a block diagram illustrating a conventional preamble acquisition apparatus according to the conventional art. Referring to FIG. 2, a correlation calculation unit 210 receives a preamble signal 201, and correlates the received preamble signal 201 and a candidate PN code 204. Here, the candidate PN code 204 is generated by a PN code generation unit 230. Also, a preamble acquisition determination unit 220 compares a correlation value 202 with a predetermined threshold value, and determines whether to acquire the preamble. When the correlation value 202 is greater than the threshold value, the PN code 204 is acquired and outputted as an acquired preamble. When the correlation value 202 is less than the predetermined threshold value, another PN code is generated by the PN code generation unit 230, and this correlation process is repeated.
Even a mobile communication system according to a conventional CDMA system uses a method for correlating a received preamble signal and a PN code in a terminal to identify a base station. In the case of the CDMA system, the length of the PN code that is used to identify the base station is 215 bits. According to this method, correlating the preamble signal and the candidate PN codes generated by the terminal requires a large number of calculations and a significant amount of time. This method also consumes scarce terminal resources to acquire the preamble. Accordingly, the entire system performance may deteriorate. Also, when the preamble is acquired by performing a large number of calculations each time the initial synchronization and the cell searching process are performed, power from a battery power source, which has only a limited power supply, may be consumed unnecessarily.
However, a systematic correlation method may reduce a preamble acquisition time. In a systematic correlation method, a preamble signal is initially correlated with a portion of a candidate PN code. If a correlation value is greater than a predetermined threshold value such that the preamble signal and the candidate PN code are determined to be sufficiently similar to each other, the preamble signal is further correlated with the remainder of the candidate PN code.
This systematic correlation method is referred to as a double dwell algorithm, and is applied to a preamble acquisition apparatus included in a CDMA mobile terminal. FIG. 3 is a block diagram illustrating the preamble acquisition apparatus using the double dwell algorithm.
Referring to FIG. 3, a local correlation calculation unit 310 correlates a received preamble signal 301 with a local PN code 302 corresponding to a portion of an entire PN code 305 generated by a PN code generation unit 350. When a correlation value 303 is greater than a predetermined threshold value, a correlation calculation is performed with respect to the received preamble signal 301 and the length of the entire PN code 305 via an entire correlation calculation unit 330. When the correlation value 303 acquired from the local correlation calculation unit 310 is less than the threshold value, a control signal 304 is transmitted to the PN code generation unit 350 to generate another PN code. Another local PN code 302 corresponding to a portion of the additionally generated PN code is then used for a local correlation with the preamble signal 301 in the local correlation calculation unit 310.
Also, when a correlation value 306 acquired from the entire correlation calculation unit 330 is less than a predetermined threshold value, a preamble acquisition determination unit 340 transmits a control signal 307 to the PN code generation unit 350 to generate an additional entire PN code 305 based upon the local PN code 302 having a correlation value 303 that is greater than a predetermined threshold value. The additionally generated entire PN code 305 is correlated in the entire correlation calculation unit 330, and acquired if the preamble acquisition determination unit 340 determines to acquire the preamble based on the correlation value 306.
According to the double dwell algorithm, a preamble acquisition time may be significantly reduced by eliminating an unnecessary correlation time. However, when using the double dwell algorithm, the calculation time may be reduced via a systematic correlation calculation, but an operation of generating a PN code is not systematically performed. Accordingly, the entire PN code 305 is generated for each calculation even when the local PN code 302, which is a portion of an entire PN code 305, results in a correlation value 303 that is less that the threshold value.
Also, a preamble acquisition apparatus using the conventional double dwell algorithm must repeat a local correlation calculation at the local correlation calculation unit 310 even when a local PN code 302, which is used for the local correlation calculation, is identical to a previously used local PN code 302.
Ineffectiveness of the double dwell algorithm, as described above, is attributed to the property of a CDMA PN code. Specifically, the length of the CDMA PN code is 215 bits, which is relatively long. Also, the CDMA PN code is not clearly defined in a standard of possible PN codes. Accordingly, performance of the double dwell algorithm may be improved by improving a method of generating entire PN codes 305, and systematically correlating a preamble signal with a portion of the entire PN codes 305.
However, in the case of the OFDMA PN code, the length of the OFDMA PN code is 284 bits, which is comparatively very short. Also, the types of possible PN codes are limited to 114. Accordingly, when appropriately using this feature of OFDMA PN codes, the problems which occur in the double dwell algorithm may be solved. Specifically, using properties of PN codes contained in OFDMA that differ from CDMA PN code properties may permit the improvement of performance over performance when using the conventional double dwell algorithm.
Accordingly, a new technology has been developed to improve preamble acquisition speed by applying a systematic correlation calculation method to an OFDMA mobile terminal, and to prevent unnecessary power consumption by leveraging the properties of OFDMA PN codes.